I think that long lasting dominant defenses may be a thing of the past due to free agency and salary caps.
While it's possible to maintain a strong defense for 3-4 seasons, it's only a matter of time when a team cannot afford to pay all the starters what they might be worth on the open market. When you look at that Cowboys defense in the early 90's, it's demise coincided with the introduction of a salary cap.
Jimmy Jones, Russel Maryland, Robert Jones, Godfrey Miles, Brock Marion, Larry Brown, Dave Thomas, Darrin Smith, and Ken Norton Jr. were all lost due to free agency and the salary cap.
Even today, when you consider the losses of players like Awuzie, Xavier Woods, Maliek Collins, Byron Jones, Damien Wilson and Anthony Hitchens, it is difficult to maintain a strong defense when you have to constantly resupply it through a draft.
Dominant past defenses such as the Doomsday, Purple People Eaters, No-Name, Fearsome Foursome, Orange Crush, and Steel Curtain were able to dominate for a decade because they didn't have to replenish their defenses to the degree that teams have to today.
The Cowboys of the late 70's also benefited from a surplus of talent by trading them in for premium draft picks. It is important to recognize that Doomsday II would not have existed without the trades that netted Too Tall Jones and Randy White, the #1 and #2 overall picks in their respected drafts.
That's not to say it's impossible and the Cowboys might be one of the very few teams that can maintain a strong defense due to the current success of the present personnel department. With that said, a team must also have a specific plan for the type of defense and players required. Today's teams must rely on early contributions from draft picks and they cannot be limited to only premium picks.
Players like DaRon Bland, Damone Clark, and Dorance Armstrong are steps in the right direction but the Cowboys cannot afford to neutralize their positive steps in the lower rounds with negative higher picks like Taco Charlton, Kelvin Joseph, and Trysten Hill.